PROJECT SUMMARY The long term goal of our research is to determine how gene expression is regulated at the post- transcriptional level. Control of messenger RNA (mRNA) translation and degradation underlies important biological processes including development, fertility and neurological functions. The proposed work focuses on the archetypal mRNA regulator, Drosophila Pumilio (Pum), which belongs to a family of RNA-binding proteins that are conserved throughout eukarya. Pum binds an extensive group of mRNAs including those that encode key developmental morphogens. Upon binding to an mRNA, Pum represses expression of the encoded protein. We find that Pum accelerates decay of the target mRNA and the proposed research seeks to discover the mechanism of Pum-mediated mRNA degradation. We developed novel assays to measure Pum activity in Drosophila cells and discovered multiple unique Repression Domains that potently repress target mRNAs. In preliminary work, we identified key co-repressors that are necessary for Pum repression. In the first aim, we measure the impact of the Repression Domains on protein expression and mRNA degradation and interrogate the functional roles of two classes of mRNA decay factors in repression by each Pum Repression Domain. We interrogate the physical interactions of the co-repressors with each Pum Repression Domain and measure their recruitment to Pum-regulated target mRNAs. In the second aim, we investigate the role of each Pum Repression Domain in embryonic development and proper spatial and temporal control of gene expression during early development. This research will reveal novel mechanisms of Pum repression. The resulting discoveries are expected to broadly enhance our understanding of post-transcriptional control and specifically improve knowledge of gene regulation in development, fertility, stem cell proliferation, and the nervous system.